Catalyst layer assembly

ABSTRACT

A reinforced catalyst layer assembly, suitably for use in a fuel cell, said reinforced catalyst layer assembly comprising:
         (i) a planar reinforcing component consisting of a porous material having pores extending through the thickness of the material in the z-direction, and   (ii) a first catalyst component comprising a first catalyst material and a first ion-conducting material,
 
characterised in that the first catalyst component is at least partially embedded within the planar reinforcing component, forming a first catalyst layer having a first surface and a second surface is disclosed.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Divisional of U.S. patent application Ser. No.13/516,914, filed Aug. 23, 2012, which is a U.S. National Phase ofInternational Patent Application No. PCT/GB2010/052092, filed Dec. 15,2010, which claims priority of British Patent Application No. 0921996.5,filed Dec. 17, 2009, the disclosures of which are incorporated herein byreference in their entirety.

FIELD OF INVENTION

The present invention relates to a reinforced catalyst layer assemblycomprising a planar reinforcing component and a first catalystcomponent, a method for the preparation thereof, and its use inelectrochemical devices, in particular fuel cells.

BACKGROUND OF THE INVENTION

A fuel cell is an electrochemical cell comprising two electrodesseparated by an electrolyte. A fuel, such as hydrogen or an alcohol,such as methanol or ethanol, is supplied to the anode and an oxidant,such as oxygen or air, is supplied to the cathode. Electrochemicalreactions occur at the electrodes, and the chemical energy of the fueland the oxidant is converted to electrical energy and heat.Electrocatalysts are used to promote the electrochemical oxidation ofthe fuel at the anode and the electrochemical reduction of oxygen at thecathode.

In proton exchange membrane (PEM) fuel cells, the electrolyte is a solidpolymeric membrane. The membrane is electronically insulating butionically conducting. In the PEM fuel cell the membrane is protonconducting, and protons, produced at the anode, are transported acrossthe membrane to the cathode, where they combine with oxygen to formwater.

The principle component of a PEM fuel cell is known as a membraneelectrode assembly (MEA) and is essentially composed of five layers. Thecentral layer is the polymer ion-conducting membrane. On either side ofthe ion-conducting membrane there is an electrocatalyst layer,containing an electrocatalyst designed for the specific electrolyticreaction. Finally, adjacent to each electrocatalyst layer there is a gasdiffusion layer. The gas diffusion layer must allow the reactants toreach the electrocatalyst layer and must conduct the electric currentthat is generated by the electrochemical reactions. Therefore, the gasdiffusion layer must be porous and electrically conducting.

Conventionally, the MEA can be constructed by a number of methodsoutlined hereinafter:

(i) The electrocatalyst layer may be applied to the gas diffusion layerto form a gas diffusion electrode. Two gas diffusion electrodes can beplaced either side of an ion-conducting membrane and laminated togetherto form the five-layer MEA;

(ii) The electrocatalyst layer may be applied to both faces of theion-conducting membrane to form a catalyst coated ion-conductingmembrane. Subsequently, gas diffusion layers are applied to both facesof the catalyst coated ion-conducting membrane.

(iii) An MEA can be formed from an ion-conducting membrane coated on oneside with an electrocatalyst layer, a gas diffusion layer adjacent tothat electrocatalyst layer, and a gas diffusion electrode on the otherside of the ion-conducting membrane.

Typically, tens or hundreds of MEAs are required to provide enough powerfor most applications, so multiple MEAs are assembled to make up a fuelcell stack. Field flow plates are used to separate the MEAs. The platesperform several functions: supplying the reactants to the MEAs; removingproducts; providing electrical connections; and providing physicalsupport.

Conventional ion-conducting membranes used in the PEM fuel cell aregenerally formed from sulphonated fully-fluorinated polymeric materials(often generically referred to as perfluorinated sulphonic acid (PFSA)ionomers). Membranes formed from these ionomers are sold under the tradenames Nafion® (e.g. NR211 or NR212 from E.I. DuPont de Nemours and Co.),Aciplex™ (Asahi Kasei) and Flemion® (Asahi Glass KK). Otherfluorinated-type membranes include those sold under the trade nameFumapem® F (e.g. F-930 or F-1030 from FuMA-Tech GmbH), Aquivion™ fromSolvay Solexis S.p.A and the GEFC-10N series from Golden Energy FuelCell Co., Ltd.

As an alternative to perfluorinated, and partly-fluorinated, polymerbased ion-conducting membranes it is possible to use ion-conductingmembranes based on non-fluorinated sulfonated or phosphonatedhydrocarbon polymers, and in particular polyarylene polymers. Suchcommercially available polymers include Udel® polyarylenesulfone (PSU)and Veradel® polyarylene ether sulphone (PES) from Solvay AdvancedPolymers, and Victrex® polyarylene ether ether ketone (PEEK™) fromVictrex plc. Hydrocarbon polymer based membranes are also described,such as the Fumapem® P, E and K types from FuMA-Tech GmbH., JHY and JEMmembranes from JSR Corporation, SPN polymer from Toyobo Co., Ltd., anddevelopmental membranes from Toray Industries Inc.

In PEM fuel cells designed to operate at higher temperatures (e.g. 150°C. to 190° C.), the membrane may be a polymer such as polybenzimidazole,or polymer matrix, impregnated with phosphoric acid. Examples of MEAsmade from such membranes include the Celtec®-P series from BASF FuelCell GmbH. Other MEAs include the Advent TPS® series based on aromaticpolyether polymers incorporating pyridine type structures, alsoimpregnated with phosphoric acid, from Advent Technologies S.A.Polybenzazole polymers can also be used such as ayrl or alkylsubstituted polybenzimidazole (e.g.polybenzimidazole-N-benzylsulfonate), polybenzoxazoles andpolybenzothiazoles.

The PFSA or hydrocarbon based ion-conducting membrane may contain areinforcement, typically wholly embedded within the membrane, to provideimproved mechanical properties such as increased tear resistance andreduced dimensional change on hydration and dehydration. The preferredreinforcement may be based on, but not exclusively, a microporous web orfibres of a fluoropolymer such as polytetrafluoroethylene (PTFE), asdescribed in U.S. Pat. No. 6,254,978, EP 0814897 and U.S. Pat. No.6,110,330, or polyvinylidene fluoride (PVDF), oralternative-materials-such as PEEK or polyethylene.

Conventionally, electrocatalyst layers are formed using well-knowntechniques, such as those disclosed in EP 0 731 520. The catalyst layercomponents may be formulated into an ink, comprising an aqueous and/ororganic solvent, optional polymeric binders and optionalproton-conducting polymer. The ink may be deposited onto anelectronically conducting gas diffusion layer or an ion-conductingmembrane using techniques such as spraying, printing and doctor blademethods.

The anode and cathode gas diffusion layers are suitably based onconventional gas diffusion substrates. Typical substrates includenon-woven papers or webs comprising a network of carbon fibres and athermoset resin binder (e.g. the TGP-H series of carbon fibre paperavailable from Toray Industries Inc., Japan or the H2315 seriesavailable from Freudenberg FCCT KG, Germany, or the Sigracet® seriesavailable from SGL Technologies GmbH, Germany or AvCarb® series fromBallard Power Systems Inc, or woven carbon cloths. The carbon paper, webor cloth may be provided with a further treatment prior to beingincorporated into a MEA either to make it more wettable (hydrophilic) ormore wet-proofed (hydrophobic). The nature of any treatments will dependon the type of fuel cell and the operating conditions that will be used.The substrate can be made more wettable by incorporation of materialssuch as amorphous carbon blacks via impregnation from liquidsuspensions, or can be made more hydrophobic by impregnating the porestructure of the substrate with a colloidal suspension of a polymer suchas PTFE or polyfluoroethylenepropylene (FEP), followed by drying andheating above the melting point of the polymer. For applications such asthe PEMFC, a microporous layer may also be applied to the gas diffusionsubstrate on the face that will contact the electrocatalyst layer. Themicroporous layer typically comprises a mixture of a carbon black and apolymer such as polytetrafluoroethylene (PTFE).

Typical electrocatalysts are selected from

-   -   (i) the platinum group metals (platinum, palladium, rhodium,        ruthenium, iridium and osmium),    -   (ii) gold or silver,    -   (iii) a base metal,

or an alloy or mixture comprising one or more of these metals or theiroxides. The metal, alloy or mixture of metal may be unsupported orsupported on a suitable support, for example particulate carbon. Theelectrocatalyst most appropriate for any given electrochemical devicewould be well known to those skilled in the art.

It has been found that using such components and constructing the MEA insuch a manner can lead to a number of problems including cracking of thecatalyst layers, which can lead to increased gas crossover, peroxideformation and thus increased membrane degradation; delamination at thecatalyst layer to membrane interface and other mechanical failures dueto expansion and contraction during wet and dry cycles experienced bythe MEA.

SUMMARY OF INVENTION

It is the object of the present invention to provide improved assembliesfor use in a fuel cell that overcome some of the problems associatedwith conventional MEA constructions.

Accordingly, a first aspect of the invention provides a reinforcedcatalyst layer assembly, suitably for use in a fuel cell, saidreinforced catalyst layer assembly comprising:

-   -   (i) a planar reinforcing component consisting of a porous        material having pores extending through the thickness of the        material in the z-direction, and    -   (ii) a first catalyst component comprising a first catalyst        material and a first ion-conducting material,        characterised in that the first catalyst component is at least        partially embedded within the planar reinforcing component,        forming a first catalyst layer having a first surface and a        second surface.

BRIEF DESCRIPTION OF THE FIGURES

This invention is illustrated by the accompanying drawings in which:

FIG. 1 is a polarization curve for a conventional membrane electrodeassembly (MEA) (data points shown as squares) and an MEA according tothe invention and as described in Example 2 (data points denoted astriangles);

FIG. 2 is an ohmic resistance curve measured by the current interrupttechnique for a conventional MEA (data points shown as squares) and anMEA according to the invention and as described in Example 2 (datapoints denoted as triangles); and

FIG. 3 is a schematic representation of the product of Example 2.

DETAILED DESCRIPTION OF THE INVENTION

The planar reinforcing component is a porous material having pores thatextend through the thickness of the material in the z-direction. In oneembodiment, the pores are discrete and not interconnected; in analternative embodiment, there is some connectivity between the pores.The pores may be regular or irregular in shape, and suitably eachindividual pore has a cross-sectional diameter of from 1 mm to 10 nm.The pores may all be of essentially similar size or there may be a rangeof sizes. The pores may have low tortuosity (i.e. the pores extendessentially in a direct route from one face to the other) oralternatively, the tortuosity of the pores is high. The dimensions ofthe planar reinforcing component in the x- and y-directions will bedependent on the size of the final product incorporating the reinforcedcatalyst layer assembly; it is well within the capabilities of theskilled person to determine the most appropriate x- and y-dimensions.The dimension of the planar reinforcing component in the z-direction maybe from 1 μm to 500 and suitably from 10 μm to 200 μm. Exact dimensionswill depend on the final structure and the use to which the reinforcedcatalyst layer assembly is put. Determination of the dimensions in thez-direction is well within the capabilities of the skilled person. Theterms ‘x-direction’, ‘y-direction’ and ‘z-direction’ are well-known tothe skilled person and meaning within the plane (x- and y-direction) andthrough the plane (z-direction).

The porosity of the planar reinforcing component is suitably greaterthan 30%, preferably greater than 50% and most preferably greater than70%. The percentage porosity is calculated according to the formulan=V_(v)/V_(t)×100, wherein n is the porosity, V_(v) is the voids volumeand V_(t) is the total volume of the planar reinforcing component. Thevoids volume and total volume of the planar reinforcing component can bedetermined by methods known to those skilled in the art.

A number of different structure types can be envisaged for the planarreinforcing component, but the pores should be connected rather thanclosed pores:

(i) A microporous structure wherein the pores are random in size andshape, but wherein the pores extend through the structure in thez-direction. Examples of such material include: polymers made porous bythe inverse phase segregation method (e.g. Ultra High Molecular WeightPolyethylene (UHMWPE) from DSM), expanded PTFE and the like, non-wovenstructures made from assemblies of fibres orientated in many directionsby wet or dry laid methods. Electrospinning of polymers can givesuitable highly porous sheets as can other methods used to manufacturefilters and the like.

(ii) A cellular structure, which comprises discrete cells, wherein thewall of each cell extends through the thickness of the material suchthat there is no inter-connection from one cell to other cells. Examplesinclude: cellular structures made by extrusion and slicing or by partiallamination of strips and subsequent expansion under tension (such asNomex® or Tyvec® from DuPont) or other means, woven meshes, meshes madeby cutting slits in continuous sheets and applying tensile stress toform expanded meshes, planar sheets with holes punched or cut (e.g. bylaser) through the thickness of the sheet, and porous sheets made bycasting material into a mould.

The planar reinforcing component may be fabricated from any material andformation that will provide the required reinforcement of the reinforcedcatalyst layer assembly. Examples of suitable material from which thereinforcing component may be made include, but are not limited to metal,carbon, ceramic and polymer. In many embodiments it is desirable for thematerial to be an electrical insulator, but in others it should be aweak electrical conductor and in some cases it does not matter if thematerial conducts or not. Preferably the material is stable in the fuelcell environment and able to form a strong bond with any polymericcomponents in the final product incorporating the reinforced catalystlayer assembly.

In some embodiments, it is envisaged that the walls or struts of theplanar reinforcing component will also be porous, on a fine scale. Thesepores are fine enough not to be penetrated by the construction materialsof the MEA and are hydrophilic enough to allow water to fill the pores.Thus in suitable fuel cells, where liquid water is present, these porescan move water through the thickness or through the plane of thereinforcement.

Preferably, the planar reinforcing component could be sourced as apreformed material, for example woven webs produced from polymerscontaining fluorine, known as fluoropolymers, such as those supplied bySefar AG (for example FLUORTEX), polytetrafluoroethylene (PTFE),ethylene tetrafluoroethylene (ETFE) or perfluoroalkoxyethylene (PFA),and microporous web structures of expanded polytetrafluoroethylene(ePTFE), such as those supplied by Donaldson Company, Inc., known asTetratex or others. Alternative fluoropolymer structures such as thosecontaining polyvinylidene fluoride (PVDF), perfluoroalkoxy(methyl vinylether) (MFA), fluorinated ethylene propylene (FEP) and a copolymer ofhexafluoropropylene and tetrafluoroethylene may be used.

The planar reinforcing component may also be sourced as a preformedmaterial such as woven webs produced from the group ofpolyaryletherketone polymers, such as those supplied by Sefar AG (forexample PEEKTEX, polyetheretherketone (PEEK)) or others. Alternativepolyaryletherketone polymer structures containing polyetherketone (PEK),polyarylether, polyaryletherketone or polyetherkeoneetherketoneketonemay be used. Alternative hydrocarbon based polymer structures which maybe used are those containing imides or amides such as polyimides,polyetherimides, polyaramides, polybenzimidazoles, sulphur containingpolymers such as poly(p-phenylene sulphide) (PPS) supplied by TorayIndustries Inc. and Ticona UK Ltd., polyethers such as polyphenyleneoxide (PPO) or polyolefins such as microporous Ultra High MolecularWeight Polyetheylene web film supplied by DSM N.V. and Lydall Inc. knownas Solupor microporous web film.

Alternatively, the planar reinforcing component could be formed in situ,using methods such as ink jet printing or gravure printing using a lowviscosity polymer dispersion or solution or monomers/oligomer to createthe desired structure. The deposited material could be crosslinked orpolymerised (using chemical or ionising radiation, UV etc.). Analternative method of producing a planar reinforcing material in situ isto use an electro-spinning technique to create an open nanofibrestructure.

The first catalyst component comprises a first catalyst material andfirst ion-conducting material, suitably a proton conducting ionomer.Examples of suitable ion-conducting materials will be known to thoseskilled in the art and are typically provided as a dispersion of theion-conducting material in a suitable liquid; examples includeperfluorosulphonic acid ionomers (e.g. Nafion® (E.I. DuPont de Nemoursand Co.), Aciplex® (Asahi Kasei), Aquivion™ (Solvay Solexis SpA),Flemion® (Asahi Glass Co.), Fumion® F-series (FuMA-Tech GmbH)), orionomers made from hydrocarbon polymers (e.g. Fumion® P-series based onpolyarylene sulphonic acid (FuMA-Tech GmbH) or phosphoric acidimpregnated polybenzimidazole (e.g. by dissolving PBI indimethylacetamide and mixing with the catalyst material and addingphosphoric acid to impregnate the PBI).

Suitable first catalyst materials are selected from

-   -   (i) the platinum group metals (platinum, palladium, rhodium,        ruthenium, iridium and osmium),    -   (ii) gold or silver,    -   (iii) a base metal,

(the ‘primary metal’) or an alloy or mixture comprising one or more ofthese primary metals or their oxides. The primary metal, alloy ormixture of metal may be unsupported or supported on a suitable support,for example particulate carbon, or electrically conducting particulateoxides. The first catalyst material most appropriate for any givenelectrochemical device would be well known to those skilled in the art.The appropriate loading of the primary metal of the first catalystmaterial in the first catalyst component is dependent on the end use andwould be known to those skilled in the art; suitably, the loading isless than 4 mg/cm², and preferably less than 2 mg/cm².

In the reinforced catalyst layer assembly of the invention, the firstcatalyst component is at least partially embedded within the planarreinforcing component such that a first catalyst layer is formed. By‘catalyst layer’, it is meant a layer in which the dimensions in the x/ydirections are considerably greater than in the z-direction so thecatalyst layer could be considered planar. The first catalyst layer hasa first surface, which is adapted to face an ion-conducting materialwhen incorporated into a membrane electrode assembly and a secondsurface adapted to face a gas diffusion layer when incorporated into amembrane electrode assembly.

In one embodiment, the first catalyst component is completely embeddedwithin the planar reinforcing component. The z-dimensions of the planarreinforcing component and the first catalyst layer may be the same orthe z-dimension of the planar reinforcing component may be greater thanthe first catalyst layer, in which case the planar reinforcing componentwill extend beyond the first catalyst layer at one or both of the firstand second surfaces of the first catalyst layer. If the planarreinforcing component extends beyond the second surface of the firstcatalyst layer, suitably it does not extend beyond the second surface bymore than 50 μm.

Alternatively, it is possible that the z-dimension of the planarreinforcing component is less than that of the catalyst layer such thatthe catalyst layer extends beyond the planar reinforcing component inthe z-direction at one or both surfaces of the planar reinforcingcomponent.

The x- and y-dimensions (‘area’) of the first catalyst layer aresuitably equal to, less than or greater than the x- and y-dimensions(‘area’) of the planar reinforcing component. In one embodiment of theinvention, the areas of the planar reinforcing component and the firstcatalyst layer are equal such that the planar reinforcing component andfirst catalyst layer are co-extensive. In a second embodiment of theinvention, the area of the first catalyst layer is less than the area ofthe planar reinforcing component, such that there is a region around theperiphery of the reinforcing component (‘first edge region’) in which nofirst catalyst layer is present. The first edge region may be at leastpartially filled with a non-ion-conducting material.

In another embodiment, the planar reinforcing component may not extendto the centre of the first catalyst layer such that only the outerregion of the first catalyst layer is reinforced, whilst the innerregion remains un-reinforced.

The reinforced catalyst layer of the invention may be made by applyingthe first catalyst component in the form of an ink to a substrate, forexample a decal transfer substrate, and while the ink is still wet,applying the planar reinforcing component such that the first catalystcomponent impregnates and becomes embedded within the planar reinforcingcomponent. The first catalyst component is then dried to form a firstcatalyst layer embedded at least partially within the planar reinforcingcomponent and the substrate removed to leave a reinforced catalyst layerassembly of the invention. By varying the thickness of the wet ink film,the top surface of the catalyst layer may be made to lie below or aboveor co-extensive with the top surface of the planar reinforcingcomponent. Alternatively, the first catalyst component in the form of anink may be sprayed into the planar reinforcing component, whilst itresides on a decal transfer substrate, or the ink may be sprayed fromboth sides such that the droplets coalesce within the pores of theplanar reinforcing component. The amount of ink added can be adjustedsuch that the surface of the dry catalyst layer stands proud of theplanar reinforcing component on one or both sides, or lies within thereinforcing component on one or both sides.

The first catalyst component may be made in the form of a dough usingsuitable rheology modifiers or by fibrillating PTFE by high shearblending within the first catalyst component. Such a dough can then becombined with the planar reinforcing component, provided the largestparticles of the first catalyst component are smaller than the smallestpores within the planar reinforcing component. Impregnation of thecatalyst dough into the planar reinforcing component can be achieved byco-extrusion, calendaring or pressing. If the face of the tool forcingthe dough into the planar reinforcing component is of suitablecompressibility, the surface of the first catalyst component can be madeto lie below the surface of the planar reinforcing component on one orboth sides.

Further the first catalyst component can be made in the form of a drypowder and applied to the planar reinforcing component by dry spraydeposition, electrostatic spray or other electrostatic methods, such asthose used in photocopiers, either with the planar reinforcing componentresting on a decal substrate or being accessible from both sides.

To combine the first catalyst component with the planar reinforcingcomponent such that the first catalyst component is wholly embedded inthe reinforcing component, the first catalyst component can be depositedinto the reinforcing component in the form of an ink from one or bothsides. Upon drying, the loss of solvent leads to shrinkage such thatboth surfaces of the first catalyst component recede within thereinforcing component. Alternatively, a thermally decomposable,sublimatable or soluble material can be added to partly fill, or blind,the pores of the reinforcing component, preferably resting on asubstrate, before the catalyst component is added, which would notcompletely fill the reinforcing component, or would shrink back duringdrying. Once the catalyst component is dry, the blinding material can beremoved by thermal decomposition, sublimation or washing.

A second aspect of the invention provides a reinforced catalyst layerassembly as hereinbefore described, and which further comprises anion-conducting component applied to the first surface of the firstcatalyst layer forming an ion-conducting layer having a first surfaceand a second surface wherein the first surface is in contact with thefirst surface of the first catalyst layer. The ion-conducting componentmay be at least partially embedded within the planar reinforcingcomponent, wholly embedded within the planar reinforcing component orwholly embedded within the planar reinforcing component such that theplanar reinforcing component extends beyond the ion-conducting componentin the z-direction.

In one embodiment, there may also be an overlap of the first catalystlayer and the ion-conducting component such that an interphase layer ofgraded composition is obtained. By this, we mean that there is a zone offinite thickness between the catalyst layer and the ion-conductingcomponent over which the composition varies progressively.

The ion-conducting component forms a layer, the layer being of similararea (x- and y-dimensions) to the area of the planar reinforcingcomponent such that the two are essentially co-extensive. Alternatively,the planar reinforcing component is of greater area than theion-conducting layer, such that there is a region around the peripheryof the reinforced catalyst layer assembly (‘second edge region’) inwhich no ion-conducting component is present. The second edge region maybe at least partially filled with a non-ion-conducting material.

The ion-conducting component is an ionomer (e.g. a perfluorosulphonicacid ionomer or hydrocarbon ionomer as hereinbefore described), or apolymer matrix impregnated or impregnatable with an electrolyte (e.g.phosphoric acid or sulphuric acid). The ion-conducting componentselected will be dependent on the ultimate use of the reinforcedcatalyst layer assembly and is within the ability of the skilled personto select the most appropriate material.

The ion-conducting component can be applied to the first catalyst layer,as a dispersion in a suitable liquid, by any technique known in the art,for example screen printing, rotary screen printing, inkjet printing,spraying, painting, immersion or dipping, bar coating, pad coating,gravure, gap coating techniques such as knife or doctor blade over roll(whereby the coating is applied to the substrate then passes through asplit between the knife and a support roller), air knife coating(whereby the coating is applied to the substrate and the excess is‘blown off’ by a powerful jet from the air knife), slot die (slot,extrusion) coating (whereby the coating is squeezed out by gravity orunder pressure via a slot onto the substrate), metering rod applicationsuch as with a Meyer bar and gravure coating. Suitably, theion-conducting component is dried to remove the liquid and form a solidfilm of the ion-conducting component.

Alternatively, the ion-conducting component may be a pre-formed polymerfilm, and it may be applied to the first catalyst layer in solid form byhot pressing in a press or hot calendar rolls.

In a yet further aspect of the invention, the reinforced catalyst layerassembly as hereinbefore described and which also comprises anion-conducting layer, may further comprise a second catalyst component,comprising a second catalyst material and a second ion-conductingmaterial, applied to the second surface of the ion-conducting layer, thesecond catalyst component forming a second catalyst layer having a firstsurface and a second surface, wherein the first surface is in contactwith the second surface of the ion-conducting layer. By ‘catalystlayer’, it is meant a layer in which the dimensions in the x/ydirections are considerably greater than in the z-direction so thecatalyst layer could be considered planar.

The second catalyst component comprises a second catalyst material and asecond ion-conducting material, suitably a proton conducting ionomer.Examples of suitable ion-conducting materials will be known to thoseskilled in the art and are typically provided as a dispersion of theion-conducting material in a suitable liquid; examples includeperfluorosulphonic acid ionomers (e.g. Nafion® (E.I. DuPont de Nemoursand Co.), Aciplex® (Asahi Kasei), Aquivion™ (Solvay Solexis SpA),Flemion® (Asahi Glass Co.), Fumion® F-series (FuMA-Tech GmbH)), ionomersmade from hydrocarbon polymers (e.g. Fumion® P-series based onpolyarylene sulphonic acid (FuMA-Tech GmbH) or phosphoric acidimpregnated polybenzimidazole (e.g. by dissolving PBI indimethylacetamide and mixing with the catalyst material and addingphosphoric acid to impregnate the PBI).

Suitable second catalyst materials are selected from

-   -   (iv) the platinum group metals (platinum, palladium, rhodium,        ruthenium, iridium and osmium),    -   (v) gold or silver,    -   (vi) a base metal,

(the ‘primary metal’) or an alloy or mixture comprising one or more ofthese primary metals or their oxides. The primary metal, alloy ormixture of metal may be unsupported or supported on a suitable support,for example particulate carbon. The second catalyst material mostappropriate for any given electrochemical device would be well known tothose skilled in the art. The appropriate loading of the primary metalof the second catalyst material in the second catalyst component isdependent on the end use and would be known to those skilled in the art;suitably, the loading is less than 4 mg/cm², and preferably less than 2mg/cm². The second catalyst component may be the same or different tothe first catalyst component.

The second catalyst component may be not embedded within the planarreinforcing component, at least partially embedded within the planarreinforcing component or wholly embedded within the planar reinforcingcomponent. If wholly embedded within the planar reinforcing component,the planar reinforcing component may extend beyond the second surface ofthe second catalyst layer in the z-direction, but suitably not by morethan 50 microns.

The second catalyst layer may be of similar area (x- and y-dimensions)to the planar reinforcing component such that the two are essentiallyco-extensive. Alternatively, the planar reinforcing component is ofgreater area than the second catalyst layer, such that there is a regionaround the periphery of the reinforced catalyst layer assembly (‘thirdedge region’) in which no second catalyst layer is present. The thirdedge region may be at least partially filled with a non-ion-conductingpolymer. Alternatively, the area of the second catalyst layer is greaterthan the area of the planar reinforcing component.

In another embodiment, the planar reinforcing component may not extendto the centre of the second catalyst layer such that only the outerregion of the second catalyst layer is reinforced, whilst the innerregion remains un-reinforced.

The second catalyst component in liquid ink form can be applied directto the ion-conducting layer by any method known to those in the art, forexample screen printing, rotary screen printing, inkjet printing,spraying, painting, immersion or dipping, bar coating, pad coating,gravure, gap coating techniques such as knife or doctor blade over roll(whereby the coating is applied to the substrate then passes through asplit between the knife and a support roller), air knife coating(whereby the coating is applied to the substrate and the excess is‘blown off’ by a powerful jet from the air knife), slot die (slot,extrusion) coating (whereby the coating is squeezed out by gravity orunder pressure via a slot onto the substrate), metering rod applicationsuch as with a Meyer bar and gravure coating. Alternatively, the secondcatalyst component may be first applied in the form of an ink to asubstrate, for example a decal transfer substrate, and while the ink isstill wet, applied to the ion-conducting layer, dried and the substrateremoved.

The second catalyst component may also be made in the form of a doughusing suitable rheology modifiers or by fibrillating PTFE by high shearblending within the first catalyst component. Such a dough can then beapplied to the ion-conducting layer by co-extrusion, calendaring orpressing.

Further the second catalyst component can be made in the form of a drypowder and applied to the ion-conducting layer by dry spray deposition,electrostatic spray or other electrostatic methods such as those used inphotocopiers.

In a still further aspect of the invention, the reinforced catalystlayer assembly as hereinbefore described further comprises a firstmicroporous layer present on the second surface of the first catalystlayer and/or a second microporous layer present on the second surface ofthe second catalyst layer. The first and/or second microporous layersmay independently be at least partially embedded within the planarreinforcing component or not embedded within the planar reinforcingcomponent. The planar reinforcing component does not extend beyond theexposed surface of the first and/or second microporous layers. The firstand/or second microporous layer independently comprises a particulatematerial, such as a carbon black, and a polymer, such as a hydrophobicpolymer. An example of a suitable polymer is for example PTFE, suitablythe PTFE is pre-fired on the carbon and the combination of PTFE andcarbon is then applied onto the first and/or second catalyst layerassembly as a dry powder or by dispersing in a suitable dispersant suchas water thickened with methyl cellulose, or an organic dispersant suchas 1-methoxy 2-propanol.

The first and second microporous layers are applied to the first andsecond catalyst layer respectively by methods known to those in the art,for example screen printing, rotary screen printing, inkjet printing,spraying, painting, immersion or dipping, bar coating, pad coating,gravure, gap coating techniques such as knife or doctor blade over roll(whereby the coating is applied to the substrate then passes through asplit between the knife and a support roller), air knife coating(whereby the coating is applied to the substrate and the excess is‘blown off’ by a powerful jet from the air knife), slot die (slot,extrusion) coating (whereby the coating is squeezed out by gravity orunder pressure via a slot onto the substrate), metering rod applicationsuch as with a Meyer bar and gravure coating.

Alternatively, the substrate onto which the reinforced catalyst layerassembly of the invention is first made could be a gas diffusion layerwhich has a first microporous layer already thereon. Depending onwhether the first microporous layer is to be at least partially embeddedwithin the planar reinforcing component, the first microporous layer isapplied to a gas diffusion substrate and either (i) the planarreinforcing component is applied to the wet first microporous layer suchthat the first microporous layer at least partially embeds within theplanar reinforcing component, the microporous layer is allowed to dryand a first catalyst layer is applied to the microporous layer such thatit embeds within the planar reinforcing layer and the first catalystlayer is allowed to dry, or (ii) the microporous layer is allowed todry, a first catalyst layer is applied to the microporous layer andwhile still wet, a planar reinforcing component is applied to the firstcatalyst layer, such that the first catalyst layer at least partiallyembeds within the planar reinforcing layer and the first catalyst layeris allowed to dry.

A yet further aspect of the invention provides a reinforced catalystlayer assembly comprising two reinforced catalyst layer assemblies eachcomprising a planar reinforcing component and a first catalyst layer andwherein at least one of the reinforced catalyst layer assemblies furthercomprises an ion-conducting layer. If only one of the two assembliescomprises an ion-conducting layer, the assemblies are combined such thatthe ion-conducting layer in the assembly comprising the ion-conductinglayer contacts the first catalyst layer of the other assembly. If bothassemblies comprise an ion-conducting layer, the assemblies are combinedsuch that the two ion-conducting layers contact each other.

The catalyst layer assemblies can be combined by methods suited to thenature of the ion-conducting components. For example, where theion-conducting component is an ionomer, joining can be by hot-pressingin a press or using hot calendar rolls or by bonding with liquidion-conducting polymer, such as a solvent or water borne dispersion, orby hot bonding with a separate sheet of solid ion-conducting polymer, bypressing or calendaring. The bonding agents, such as the liquid ionconducting polymer or separate sheet of ion conducting polymer, may beof different composition to the ion-conducting component within thecatalyst layer assemblies. For example, a hydrocarbon ion conductingbonding agent may be used between PFSA ion-conducting polymers to reducethe amount of gas crossover, since hydrocarbon ionomers are less gaspermeable than PFSA ionomers.

The reinforced catalyst layer assemblies of the invention may furthercomprise an additive. The additive may be present at one or more of theinterfaces between the various layers and/or within one or more of thelayers.

The additive may be one or more selected from the group consisting ofhydrogen peroxide decomposition catalysts, radical scavengers, freeradical decomposition catalyst, self regenerating antioxidant, hydrogendonors (H-donor) primary antioxidant, free radical scavenger secondaryantioxidant, oxygen absorbers (oxygen scavenger). Examples of thesedifferent additives may be found in WO2009/040571 and WO2009/109780. Apreferred additive is cerium dioxide (ceria).

In a preferred embodiment of the invention, the reinforced catalystlayer assembly is used in a fuel cell, in particular a PEM fuel cell.

Accordingly, a further aspect of the invention provides a gas diffusionelectrode comprising a gas diffusion layer and a reinforced catalystlayer assembly according to the invention. The gas diffusion layer maybe a conventional gas diffusion layer as typically used in the art anddescribed hereinbefore.

A further aspect of the invention provides a reinforced catalyst layermembrane assembly comprising a reinforced catalyst layer assembly of theinvention and an ion-conducting membrane. The ion-conducting membrane isa preformed membrane and may be a conventional membrane as used in theart and described hereinbefore. The reinforced catalyst layer membraneassembly may comprise a second reinforced catalyst layer assembly of theinvention wherein the ion-conducting membrane is sandwiched between thetwo reinforced catalyst layer assemblies.

A still further aspect of the invention provides a membrane electrodeassembly comprising a reinforced catalyst layer assembly, a gasdiffusion electrode or a reinforced catalyst layer membrane assembly ofthe invention.

The membrane electrode assembly may further comprise components thatseal and/or reinforce the edge regions of the membrane electrodeassembly for example as described in WO2005/020356. The MEA is assembledby conventional methods known to those skilled in the art, for exampleas described hereinbefore.

A yet further aspect of the invention provides a fuel cell comprising areinforced catalyst layer assembly, a gas diffusion electrode, areinforced catalyst layer membrane assembly or a membrane electrodeassembly as hereinbefore described.

The reinforced catalyst layer assembly and other components of theinvention provide a number of improvements over the state-of-the-artproducts. Deformation in both the xy plane and the z plane will bereduced, which will lead to reduced cracking of the layers and thereforereduced crossover. Furthermore, mechanical failure will be reduced sinceexpansion and contraction of the final product incorporating thereinforced catalyst layer assembly will be minimised because of theconstraining effect of the reinforcement. Furthermore, delamination atinterfaces, which is seen in many state-of-the-art products, will beless of, or no longer be, a problem.

Whilst the invention has been described primarily with reference to PEMfuel cells, the invention could equally well be used, with no or littlemodification, in other types of fuel cell or electrochemical device. Forexample, in phosphoric acid fuel cells (PAFCs) the two electrodes areconventionally separated by a matrix of particulate material, such assilicon carbide bound with polyether sulphone which has a porosity ofgreater than 38% (P. Stonehart and D. Wheeler, Modern Aspects ofElectrochemistry, Number 38, edited by B. E. Conway et al, KluwerAcademic/Plenum Publishers, New York, 2005, p.400 et seq). The presentinvention allows the matrix to be replaced by the porous reinforcingmaterial, impregnated with phosphoric acid, provided a material ofsuitable resistance to hot phosphoric acid and wettable by phosphoricacid is used, such as silicon carbide, polyether sulphone, PEEK, poorlyconducting carbon fibres and other materials shown to have suitablestability.

In alkaline fuel cells the electrolyte is typically strong sodium orpotassium hydroxide and the electrodes are separated by a matrix in asimilar manner to that described for PAFCs. The present invention isquite suitable for MEAs for use in alkaline fuel cells provided theporous reinforcing component is stable in and wetted by the alkalineelectrolyte.

All of the embodiments described for PEM fuel cells apply equally toMEAs for PEM electrolysers. In these PEM electrolysers, a voltage isapplied across the membrane electrode assemblies such that watersupplied to the device is split into hydrogen and oxygen, at the cathodeand anode respectively. The MEAs may require different catalystcomponents to a PEM fuel cell, such as Ir and Ru based materials at theanode, but are otherwise very similar to MEAs used for fuel cells.

EXAMPLES

The invention will now be further described with reference to thefollowing example, which is illustrative, but not limiting of theinvention.

Example 1: A reinforced catalyst layer membrane electrode assembly wascreated by a first step of hot pressing a 30 micron thick PFSA membrane(SH-30 from Asahi Glass) into a 50 micron thick woven PEEK (Sefar)reinforcement, with an open area (porosity) of 58%, at 150° C. and 1600psi for 2 minutes. Because of the difference in thickness, the ionomercomponent was located to one side of the reinforcing layer, leavingapproximately 20 microns of the woven material un-impregnated. Catalystlayers were added by hot pressing dried catalyst layers, supported onPTFE decal transfer substrates, to both sides of the ionomer-impregnatedreinforcement at 150° C. and 800 psi for 2 minutes. In this way, anassembly was created with reinforced catalyst layers separated by anion-conducting component.

The assembly was tested in a fuel cell by combining with gas diffusionlayers, coated with microporous layers, and sealing into a test cell of50 cm² active area. Hydrogen was supplied to the anode side and oxygento the cathode side at a pressure of 100 kPa gauge and both gases werefully humidified. A polarisation curve was determined as shown in FIG. 1and the ohmic resistance of the assembly determined using thecurrent-interrupt method, as shown in FIG. 2. The cell voltage of theMEA containing the reinforced catalyst layer assembly was essentiallythe same as that of the conventional MEA, when corrected for ohmic (iR)losses. The ohmic losses were higher for the reinforced MEA by a factorof about two, consistent with the approximately 50% open area of thereinforcement.

Example 2: A reinforced membrane electrode assembly was created by firstcoating a thin layer of electrocatalyst ink, using a bar coater, ontoskive PTFE which was subsequently oven dried at 105° C. A secondarylayer of electrocatalyst ink was cast onto the pre-existing dry layer;whilst wet, a 15-20 micron thick expanded-PTFE web was laid into thissecondary ink layer. Visible penetration of the ink into the e-PTFEmatrix was observed. The interim assembly was again oven dried at 105°C. At this stage, the e-PTFE reinforcement material remained proud ofthe catalyst layer at one surface. An ion-conducting layer was thenconstructed by casting three ionomer layers onto the surface of thecatalyst layer from which the e-PTFE was proud, with oven drying aftereach layer was applied. To complete the membrane electrode assembly, afurther two coats of the electrocatalyst ink were applied to theassembly (with oven drying at each stage) resulting in the secondcatalyst layer. In the final product, the e-PTFE reinforcement materialwas entirely covered by the final electrocatalyst layer. In this way, anassembly was created in which the electrodes (anode and cathode),ionomer and electrode/ionomer interfaces of the MEA were reinforced by acontinuous e-PTFE structure (see FIG. 3).

1. A process for preparing a reinforced catalyst layer assembly suitable for use in a fuel cell, said reinforced catalyst layer assembly comprising: (a) a planar reinforcing component consisting of a porous material having pores extending through the thickness of the material in the z-direction, the planar reinforcing component having a first surface and a second surface; (b) a first catalyst layer having a first surface and a second surface, the first catalyst layer comprising a first ion-conducting material and a first primary metal; (c) a second catalyst layer having a first surface and a second surface, the second catalyst layer comprising a second ion-conducting material and a second primary metal; (d) an ion-conducting layer having a first surface and a second surface, wherein the first surface of the ion-conducting layer is in contact with the first surface of the first catalyst layer, and the second surface of the ion-conducting layer is in contact with the first surface of the second catalyst layer; wherein the planar reinforcing component extends across the ion-conducting layer into the first catalyst layer, such that the first surface of the planar reinforcing component is within the first catalyst layer; wherein said process comprises the steps: (i) applying a coating of a first ink comprising the first catalyst component onto a substrate; (ii) applying the planar reinforcing component onto the coating of the first ink, such that the first ink partially impregnates and becomes embedded within the planar reinforcing component to form the first catalyst layer, wherein the first catalyst layer is partially embedded within the planar reinforcing component such that the first surface of the planar reinforcing component is within the first catalyst layer and the second surface of the planar reinforcing component extends beyond the first surface of the first catalyst layer in a through-plane direction; (iii) applying a dispersion comprising a third ion-conducting material to the first surface of the first catalyst layer, such that the third ion-conducting material at least partially impregnates and becomes embedded within the planar reinforcing component extending beyond the first surface of the first catalyst layer to form the ion-conducting layer, wherein the ion-conducting layer is at least partially embedded within the planar reinforcing component; (iv) applying a coating of a second ink comprising the second catalyst component to the second surface of the ion-conducting layer; (v) drying to form the reinforced catalyst layer assembly.
 2. The process according to claim 1, wherein: in step (iii) the third ion-conducting material fully impregnates and becomes embedded with a portion of the planar reinforcing component extending beyond the first surface of the first catalyst layer, such that the ion-conducting layer is fully embedded within the planar reinforcing component, and wherein the second surface of the planar reinforcing component extends beyond the second surface of the ion-conducting layer in a through-plane direction; in step (iv), the second ink partially impregnates and becomes embedded within the planar reinforcing component extending beyond the second surface of the ion-conducting layer, such that the second surface of the planar reinforcing component is within the second catalyst layer.
 3. The process according to claim 1, wherein step (ii) further includes a drying step to dry the first ink.
 4. The process according to claim 2, wherein step (ii) further includes a drying step to dry the first ink.
 5. The process according to claim 1, wherein step (iii) further includes a drying step to dry the third ion-conducting material.
 6. The process according to claim 2, wherein step (iii) further includes a drying step to dry the third ion-conducting material.
 7. The process according to claim 1, wherein the substrate is removed after step (v).
 8. The process according to claim 2, wherein the substrate is removed after step (v).
 9. The process according to claim 1, wherein the substrate is a gas diffusion layer comprising a gas diffusion substrate and a microporous layer, wherein the microporous layer is adjacent to the first catalyst layer.
 10. The process according to claim 2, wherein the substrate is a gas diffusion layer comprising a gas diffusion substrate and a microporous layer, wherein the microporous layer is adjacent to the first catalyst layer.
 11. The process according to claim 1, wherein a microporous layer is applied to the second surface of the second catalyst layer.
 12. The process according to claim 2, wherein a microporous layer is applied to the second surface of the second catalyst layer.
 13. The process according to claim 1, wherein the planar reinforcing component is a cellular structure comprising discrete cells wherein the wall of each cell extends through the thickness of the material.
 14. The process according to claim 2, wherein the planar reinforcing component is a cellular structure comprising discrete cells wherein the wall of each cell extends through the thickness of the material. 